It was reported that a room temperature continuous-wave operation of a GaInP/AlGaInP semiconductor laser was realized at a 0.68 .mu.m band. Since then, a development of the semiconductor laser has been intensively promoted, and a life of several thousands hours was confirmed to increase a reliability so that a possibility of practical use thereof is sharply increased. In expectation of improved properties of the semiconductor laser, it will be demanded in many fields such as an optical information processing means including a compact disk, an optical disk, a video disk, POS etc. , and an optical communication or physical measurement system including an optical fiber. Simultaneously, it will be required that the semiconductor laser is improved even in a property of a transverse mode thereof. Especially, a semiconductor laser with a low noise property is demanded in a field of processing analog signal like a video disk, while a spectrum unity is required in a field of an optical communication system. In these respects, a semiconductor laser of a refractive index-guided type, for instance, a buried heterostructure is considered to be most pertinent as a transverse mode controllable structure to meet the aforementioned properties.
A structure of the GaInP/AlGaInP semiconductor laser which provides a high reliablity is of a gain-guided type including a mechanism for narrowing current therein, or of a loss-guided type including a semiconductor layer of a large absorption coefficient provided on side walls of a mesa as described later even if the latter is of a transverse mode controllable structure.
In such semiconductor lasers of gain-guided type or a loss-guided type, a spectra of a multi-mode oscillation are obtained to result in a degradation of S/N ratio. Therefore, it is considered that a feedback noise characteristic i badly affected as compared to a semiconductor laser of a refractive index-guided type in which a high frequency bias method is adopted. Further, astigmatism is large because a plane of guided wave is curved so that the increase of recording density is difficult on an optical disk. For these reasons, a semiconductor laser of a refractive index-guided type can be advantageously used for a transverse mode controllable structure which is applied to an optical information processing or communication system.
Therefore, a semiconductor laser of a refractive index-guided type will be mainly considered hereinafter. However, there is no report so far in which a GaInP/AlGaInP semiconductor laser of a refractive index-guided type is discussed as a tranverse mode controllable structure. For the reason, semiconductor lasers of three types which are easily conceived from a conventional GaAs/AlGaAs semiconductor laser of a refractive index-guided structure will be discussed, provided that a transverse mode controllable structure peculiar to a liquid phase epitaxy (LPE) process like a V-channeled substrate inner stripe laser, a channeled substrate planar laser etc. is not considered herein because GaInP/AlGaInP semiconductor material is not grown by the LPE process for the reason why a segregation constant of aluminum is very large so that the growth thereof is performed only by a metalorganic vapour phase epitaxy (MOVPE) process or a molecular beam epitaxy (MBE) process.
A first type is a semiconducter laser of a loss-guided type in which a current blocking layer is GaAs, and a cladding layer is partially thin to provide a mesa buried with the current blocking layer, and which is reported that a room temperature continuous oscillation is realized therein.
In the semiconducter laser, it is assumed that the current blocking layer is of AlGaInP or AlInP which is larger in a composition of aluminum than AlGaInP for the cladding layer having the mesa thereby providing a semiconducter laser of a refractive index-guided type.
In the assumed semiconducter laser, however, there is expected disadvantage that the fabrication of the semiconducter laser is not smoothly performed because lump polycrystal is percipated on a selective mask of, for instance, SiNx film in a case where the mesa is buried with AlGaInP or AlInP by a selective epitaxy of the MOVPE process wherein the percipation of the lump polycrystal is increased in proportional to a value of "X" in (Al.sub.X Ga.sub.1-X).sub.Y In.sub.1-Y P.
A second type is a buried heterostructure laser of GaAs/AlGaAs which comprises a narrow active layer sandwiched by narrow cladding layers, and two current blocking layers grown on both side walls, and which is fabricated by a normal LPE process including at least two separate steps.
In the buried heterostructure laser, there is expected a disadvantage that an incomplete growth of the current blocking layers is resulted on both side walls of the active layer and the cladding layers to decrease a reliability thereof in a case where the current blocking layers ar of AlGaInP crystal because both side walls of the active layer and the cladding layers are exposed to the atmosphere and the crystal is very sensitive to a contamination of O.sub.2. Further, leakage current tends to flow when boundary surfaces of the current blocking layers are deviated from the active layer.
A third type is a semiconducter laser of a self-aligned type which is fabricated by a normal MOVPE process in a case where GaAs/AlGaAs semiconducter materials are used therein. The semiconducter laser is assumed that a current blocking layer is of AlGaInP or AlInP which is larger in composition of aluminum than that of AlGaInP for cladding layers in a current injecting region to provide a real refractive index difference.
In the assumed semiconducter laser, there are expected disadvantages that a cladding layer is grown incompletely in a second MOVPE process on a first MOVPE-grown cladding layer, and a crystal quality thereof is degraded because the current blocking layer grown on the cladding layer by a first MOVPE process is provided with an aperture to expose a portion of the atmosphere thereby resulting in an oxidation thereof, and cladding and cap layers are then grown thereon by a second MOVPE process, and that a resistance of boundary surfaces between the cladding layers is increased for the same reason described above. Further, a deterioration of the semiconductor laser is promoted to decrease a reliability for the reason why light density is high at the boundary surfaces and current is flowed therethrough because the boundary surfaces are very near the active layer.